This invention relates to a type head and more particularly to a type head for use in a motor-controlled impact serial printer in which a plurality of type units are arranged on a circle.
Most of the prior art impact serial printer are mechanical printers in which the printing operation is performed by distributing a common power among type unit selection, line feed and other control operations by using such complicated and precise mechanical elements as cams, levers, clutches, etc. However, such mechanical printers are not advantageous from the standpoints of their operating speed, weight and price so that it is a recent trend to substitute motor-controlled impact serial printers for the mechanical printers. This invention is directed to the motor-controlled impact serial printers wherein the various printing operations described above are performed directly by individual control motors.
The type heads which have been used in the prior art motor-controlled impact serial printers can be classified into boxed type heads, and petal or daisy type heads. In the former, each of a plurality of type units is movably contained in each of a plurality of openings provided in a rigid casing, and the back of a selected type unit is hit by a printing hammer at the time of printing thereby printing a charactor on paper disposed in front of the type unit. Since each such type unit is operated independently of adjacent type units, side printing can be positively prevented. However, there are many sliding surfaces which become worn, and as a result the reliability of the boxed type head is low. Moreover, as there are a number of component parts, it requires a complicated assembling operation thus increasing the cost of manufacturing. As shown in FIG. 1, in the daisy type head a plurality of type supporting arms 13 having the same length and extending in the radial direction are secured to a hub 12 mounted on the rotary shaft of a control motor, and a type unit 14 is integrally mounted on the outer end of each supporting arm. In such device, the printing is performed by striking a selected type head by a printing hammer so to flex the supporting arm. The daisy type head has a simpler construction and requires a lesser number of component parts than the box type and is thus highly suited for mass production. However, since the daisy type units are arranged in a plane, the rotational moment of inertia increases as the square of the number of type units. For this reason, there is a limit on the number of the type units. For example, the printer presently used most widely utilizes less than 96 characters.
Where it is necessary to use at least 128 characters, for example, as in the case of type units having Japanese characters, the rotational moment of inertia increases greatly, thereby requiring that the operating speed be decreased. Further, when the number of the type units increase, the length of the supporting arms increase thus creating undesirable vibrations (resonance) resulting in the degradation of the quality of the printed matter. Where the type head is manufactured with plastics, as the number of the type units increases, it is necessary to use complicated and precise metal molds. Such metal molds have a relatively short life and are thus not suitable for mass production. Although it is possible to mount a plurality of type units on each supporting arm without increasing the length thereof, such construction is likely to cause side printing.